Conventional cationic surface sizing agents usually contain styrenes and a monomer having a tertiary amino group (i.e., a cationic monomer) as a main component. The sizing agent is an aqueous solution of a copolymer having a tertiary amino group obtained by polymerizing the monomer component, or a copolymer having a quaternary ammonium salt group obtained by quaternizing the tertiary amino group (Patent Documents 1 and 2). These copolymers are prepared by the following method.
(i) Solution-polymerization using an oil-soluble polymerization catalyst is performed in an organic solvent or a mixed solvent of an organic solvent and water.
(ii) Emulsion polymerization using a water-soluble polymerization catalyst is performed in a water-based solvent.
The cationic surface sizing agent obtained by such a method has a good sizing property, but has a problem that it is susceptible to environment of usage, especially influences of water (hardness, pH, etc.) and its stability is insufficient.
Further, as the cationic surface sizing agent obtained by polymerization in a solution, a sizing agent using a water-soluble polymer in combination is known (Patent Document 3). However, as with the surface sizing agent described in Patent Documents 1 and 2, this surface sizing agent has a good sizing property, but it is susceptible to environment of usage, especially influences of water (hardness, pH, etc.) and its stability is insufficient.
On the other hand, in the cationic surface sizing agent obtained by performing emulsion polymerization in water without using an organic solvent, or in the cationic surface sizing agent obtained by performing emulsion polymerization of a cationic polymer obtained by solution-polymerization and a hydrophobic monomer, a surfactant is often used as an emulsifier (Patent Documents 4 to 8). An emulsion polymerization product using the surfactant is less susceptible to an impact of environment of usage than the cationic surface sizing agent obtained by the solution-polymerization, and an improvement in stability can be expected, but the improvement effect is not adequate. Furthermore, such emulsion polymerization product is inferior in the effect of imparting a sizing property to the cationic surface sizing agent obtained by the solution-polymerization because of use of a surfactant.
Further, a production method in which emulsion polymerization is performed without using the surfactant is known (Patent Documents 9 and 10). In the cationic surface sizing agent obtained without using the surfactant, an aqueous solution of a cationic polymer obtained by solution-polymerization is used as a dispersing agent. Hence, the cationic surface sizing agent exhibits the stability equal to and the sizing property higher than the sizing agent using the surfactant. However, the cationic surface sizing agent obtained by the emulsion polymerization is inferior in the effect of imparting a sizing property to the cationic surface sizing agent obtained by the solution-polymerization.